1. Field of the Invention
The present invention relates to a tuning voltage generating apparatus. More specifically, the present invention relates to a tuning voltage generating apparatus of a voltage synthesizer type for supplying a tuning voltage to a tuner employing a variable reactance element as a tuning element.
2. Description of the Prior Art
FIG. 1 is a block diagram showing a major portion of a conventional television receiver. Since such television receiver is well-known to those skilled in the art, only those portions associated with the present invention will be briefly described. A tuner 100 comprises two input terminals 117 and 119. The input terminal 117 is connected to receive a television signal received by a VHF antenna 1. The input terminal 119 is connected to receive a television signal received by a UHF antenna 2. The received signal from the VHF antenna input terminal 117 is applied to a VHF high frequency amplifier 103 and is amplified and the amplified output therefrom is applied to a VHF mixer 105. The tuner 100 also comprises a VHF local oscillator 107. The oscillation output of the VHF local oscillator 107 is applied to a VHF mixer 105. Accordingly, the VHF mixer 105 serves to mix the VHF television signal with the oscillation output from the VHF local oscillator 107, thereby to convert the VHF television signal into a VHF intermediate frequency signal. On the other hand, the received signal applied to the UHF antenna input terminal 119 is applied to a UHF high frequency amplifier 109 and is amplified and the amplified output therefrom is applied to a UHF mixer 111. The tuner 100 also comprises a UHF local oscillator 113 and the oscillation output therefrom is applied to a UHF mixer 111. Accordingly, the UHF mixer 111 serves to mix the UHF television signal with the oscillation output from the UHF local oscillator 113, thereby to convert the UHF television signal into a UHF intermediate frequency signal. The output from the UHF mixer 111, i.e. the UHF intermediate frequency signal is amplified by a UHF intermediate frequency amplifier 115 and is applied to a VHF mixer 105. On the occasion of reception of the UHF signal, the VHF high frequency amplifier 103 and the VHF local oscillator 107 are disabled, while the VHF mixer 105 is kept enabled. Accordingly, on the occasion of reception of the UHF signal, the VHF mixer 105 serves as a UHF intermediate frequency amplifier for amplifying the UHF intermediate frequency signal. Meanwhile, on the occasion of reception of the VHF signal, those circuits 109, 111, 113 and 115 associated with the UHF signal are all disabled, while only those circuits 103, 105 and 107 associated with the VFH signal are enabled. The VHF intermediate frequency signal or the UHF intermediate frequency signal obtained from the VHF mixer 105 is applied from the output terminal 121 to the subsequent stage intermediate frequency circuit, not shown. These circuits 103 to 115 are housed within a shield member 101 of such as a metallic casing or frame. Therefore, any undesired radiation from those circuits housed within the shield member 101 toward other wireless equipment is effectively prevented, while any undesired electric wave or interference electric wave from other wireless equipment to those circuits is also effectively prevented. The above described antenna input terminals 117 and 119 and the intermediate frequency output terminal 121 are formed at predetermined positions of the shield member 101, while these terminals are electrically isolated from the shield member 101.
The VHF high frequency amplifier 103, the VHF local oscillator 107, the UHF high frequency amplifier 109 and the UHF local oscillator 113 each comprise a tuning circuit, not shown, for varying the tuning frequency for selection of a desired channel within a desired receiving frequency band. Each of these tuning circuits comprises a voltage controlled variable reactance device such as a voltage controlled variable capacitance diode. To that end, the tuner 100 housed in the shield member 101 is also provided with a tuning voltage input terminal 123, as electrically isolated from the shield member 101, for supply of the tuning voltage Vt. The tuning voltage Vt from the terminal 123 is applied to the associated circuits 103, 107, 109 and 113. The shield member 101, i.e. the tuner 100, further comprises a test point (TP) terminal 127, as electrically isolated from the shield member 101, for supply of the output from the tuner 100 to alignment equipment, not shown, for alignment of the output waveform on the occasion of adjustment of the tuner 100. In general, the VHF band comprises a VHF low band (the first band) of a relatively low frequency range and a VHF high band (the second band) of a relatively high frequency range. On the other hand, the UHF band may be considered as the third band of a frequency range higher than that of the VHF high band. Accordingly, the tuner 100 further comprises terminals 129, 131 and 133, as electrically isolated from the shield member 101, for supply of voltage signals for selection of these frequency bands. More specifically, the terminal 129 is aimed to supply a band selection voltage BL for selection of the VHF low band, the terminal 131 is aimed to provide a band selection voltage BH for selection of the VHF high band, and the terminal 133 is aimed to provide a band selection voltage BU for selection of the UHF band. The tuner 100 further comprises a terminal 125 for supply of an automatic gain control (AGC) voltage obtained from the intermediate frequency circuit, not shown, and a terminal 135 for supply of an automatic fine tuning (AFT) voltage, both electrically isolated from the shield member 101. Each of the terminals 129, 131 and 133 is supplied with the band selection voltage BL, BH or BU of +15 V, when the corresponding receiving frequency band is to be selected. Each of the tuning circuits included in the tuner 100 is structured to be responsive to the given band selection voltage BL, BH or BU to change the circuit constant or circuit connection of the tuning scheme so as to be adaptable to the corresponding frequency band, as well-known to those skilled in the art.
The above described tuning voltage Vt is supplied from a tuning voltage generating circuit 4. The band selection voltage BL, BH or BU is supplied from a band selection voltage generating circuit 5. The tuning voltage generating circuit 4 and the band selection voltage generating circuit 5 are supplied with a signal from a channel selector 3. The channel selector 3 includes, for example, a keyboard, a necessary operating switch and the like (not shown). The channel selector 3 further comprises a PWM wave generating circuit 31 (FIG. 2), control means, not shown, for controlling the same, and the like. If and when the keyboard (not shown), is operated so as to select a desired channel, an input pulse signal having a pulse width corresponding to that channel, that is, a frequency to be received is generated from the PWM wave generating circuit 31. At the same time, the band selection voltage circuit 5 generates the band selection voltage BL, BH or BU so as to enable the receiving band belonging to the desired channel. The tuning voltage generating circuit 4 includes a voltage converter 41 and a low-pass filter 42 as shown in FIG. 2. As a result, the tuning voltage generating apparatus in FIG. 1 is structured to be of a voltage synthesizer type.
Such a tuning voltage generating apparatus of a voltage synthesizer type is well-known to those skilled in the art and, for example, is disclosed in U.S. Pat. No. 3,968,440 issued to George John Ehni, III on July 6, 1976 and the like. Briefly stated, the voltage converter 41 comprises a transistor 411, a resistor 412 connected to the collector of the transistor 411 and a constant voltage diode 413. An input pulse signal from an output terminal 31a of the PWM wave generating circuit 31 has a pulse width corresponding to a desired channel (FIG. 3A). Such an input pulse signal is converted into a higher level pulse signal as shown in FIG. 3B. At this time, the resistor 412 and the constant voltage diode 413 cooperate with each other to restrict the collector voltage of the transistor 411 to, for example, approximately 33 V. The output of the low-pass filter 42 having the output voltage from the voltage converting circuit 41 is used as a tuning voltage Vt of a direct current voltage, as shown in FIG. 3D. FIG. 3C shows a voltage waveform in the point 42a included within the filter 42.
A so-called autosearch operation as shown in FIG. 4A and FIG. 4B can be made by using the tuning voltage generating apparatus as shown in FIG. 1 and FIG. 2. In an autosearch mode, the band selection voltage generating circuit 5 generates the band selection voltage BL, BH or BU as shown in FIG. 1 and FIG. 4B. On the other hand, the PWM wave generating circuit 31 (FIG. 2) applies to the tuning voltage generating circuit 4 an input pulse signal the pulse width of which sequentially becomes wider. By doing so, a gradually increasing tuning voltage Vt as shown in FIG. 4A is obtained for each band from the tuning voltage generating circuit 4. As a result, a tuning frequency of a tuning element, not shown, included in the tuner 100 gradually becomes higher and a desired autosearch operation can be achieved.
The above described tuning voltage Vt was determined in accordance with the following condition. In the following, therefore, such determination of the tuning voltage will be described with reference to an example of a television tuner in Europe, as shown in FIG. 5. Referring to FIG. 5, the abscissa indicates the tuning voltage and the ordinate indicates the respective channels in the VHF low band, the VHF high band and the UHF band. In Europe, for example, the VHF low band (the first band) covers channels E2 to E4, while the VHF high band (the second band) covers channels E5 to E12. The UHF band (the third band) covers channels E21 to E69. Such tuner has been designed such that the lower limit frequency of the VHF low band may be determined so that channel E2 can be received when the tuning voltage Vt is 3 V, for example. However, a television tuner must be capable of surely selecting channel E2 even in any situation and even in the worst condition. More specifically, in consideration of a frequency drift due to a source voltage fluctuation, an ambient temperature variation, a time dependent change and so on, a frequency deviation due to a mechanical shock, and the like, the television tuner must be designed to be capable of surely receiving channel E2 even in the worst condition which seldom occurs. Therefore, according to a conventional approach, the tuner 100 was designed such that the tuning voltage Vt which is as low as 0.2 to 0.3 V, for example, and is sufficiently lower than the above described 3 V, may be supplied from the tuning voltage generating circuit 4. As a result, with such a conventional television tuner, the receivable frequency range extended over the lower region beyond the necessary receivable frequency range shown by the dotted line in FIG. 5 in a normal use condition. For example, a conventional tuner was adapted such that in the case of the VHF low band shown in FIG. 5 the signal can be received even when the frequency becomes lower than that of channel E2 by a frequency difference corresponding to approximately one channel. A conventional tuner was further adapted such that in the case of the VHF high band the signal can be received even when the frequency becomes lower than the lower limit channel E5 by a frequency difference corresponding to approximately six channels. A conventional tuner was further adapted such that in the case of the UHF band the signal can be received even when the frequency becomes lower than the lower limit channel E21 by a frequency difference corresponding to approximately ten channels. A conventional television tuner was further adapted such that as for the upper limit of the respective bands as well the signal of any desired receiving frequency band can be surely received with a sufficient margin in full consideration of any imaginable worst condition. The consideration is paid such that the margin is approximately two channels over E4 channel in the case of VHF low band, approximately three channels over E13 channel in the case of VHF high band and approximately ten channels over E69 channel in the case of UHF band, respectively.
However, for the purpose of effective utilization of the electric wave and observance of secrecy of communication, in some countries there have been tendencies to restriction of reception by a tuner beyond the receivable frequency range in a television receiver, for example. More specifically, some countries have shown tendencies to legislation to restrict the receivable frequency range by a tuner in a television receiver at the upper and lower limits of the respective receiving frequency bands as shown in FIG. 5, with a margin frequency corresponding to one channel, respectively.
For example, in West Germany, the FTZ (Fermnelde Technisches Zentralamt) has made the following proposal in the draft of January, 1979. More specifically, in West Germany the frequency range for the television broadcasting has been determined such that the Band I covers 47 MHz to 68 MHz, the Band III covers 174 MHz to 230 MHz and the Band IV and V cover 470 MHz to 790 MHz. A deviation allowance outside the frequency range at each of the upper and lower limits of the frequency range of each band has been determined in principle as 300 kHz. By way of an exception, as for the receiving frequency band of 47 MHz to 870 MHz, a deviation allowance outside the frequency range has been determined as 7 MHz at the lower limit of the frequency range and as 8 MHz at the upper limit of the frequency range.
An attempt has also been made to make similar restriction in the case of the Canadian television broadcasting shown in FIG. 6. According to the Canadian television broadcasting standard, the VHF low band comprises Channel Nos. 2 to 6, the VHF high band comprises Channel Nos. 7 to 13, and the UHF band comprises Channel Nos. 14 to 84. According to the draft of October, 1978 by the Canadian DOC (Department of Communications) and the further developments thereof, the following restriction has been planned. More specifically, according to the Canadian television broadcasting standard, the channels for the CATV have been allotted in the region lower than Channel No. A7 and in the region higher than Channel No. 13. Therefore, a restriction has been planned in Canadian television receivers such that some of the CATV channels allotted in the region lower than Channel No. A7 and in the region higher than Channel No. A13 are made absolutely unreceivable. More specifically, television receivers originally not designed to receive such CATV broadcasting are sufficient enough to be capable of surely receiving only the television signal of Channel Nos. A2 to A6, Nos. A7 to A13, and Nos. A14 to A83 and therefore a restriction has been planned to make such receivers incapable of receiving a signal in Channels A to I of the CATV channels in the region lower than Channel No. A7 and a signal in CATV Channels A to W in the region higher than Channel No. A13. In making such restriction, however, one channel, i.e. Channel I in the region immediately lower than Channel No. A7 and one channel, i.e. Channel J in the region immediately higher than Channel No. A13 have been considered as allowable for a deviation range.
As described in the foregoing, in some countries there have been tendencies to a strict restriction to a deviation downward or upward from the original receiving frequency band, for the purpose of effective utilization of an electric wave and observance of communication secrecy.
The above described upper limit of receiving frequency is determined depending on variation range of the tuning voltage Vt. In a conventional tuning voltage generating apparatus of a voltage synthesizer type as shown in FIG. 2, a variation range of the tuning voltage and thus the upper limit thereof is determined depending on the constant voltage diode 413. Such constant voltage diode 413 requires good temperature characteristic and as a result, those put into practical use based on current technical level are .mu.PC-574J manufactured by Nippon Electric Co., Ltd. and the like. The operating voltage of this .mu.PC-574J may be diversified in the range of 33 V.+-.3 V according to the standard thereof. More specifically, the operating voltage of the constant voltage diode 413 such as .mu.PC-574J causes diversification in the range of 30 V to 36 V. Accordingly, the upper limit of the tuning voltage from the tuning voltage generating circuit as shown in FIG. 2 correspondingly diversifies.
Therefore, for example, assuming a television tuner for only West Germany, the standard of FTZ can be met in the diversification range of the operating voltage of the above described constant voltage diode 413 if the tuner 100 is preset or adjusted so that the E65 channel, for example, can be received at the upper limit of the tuning voltage Vt.
However, in a television receiver for European countries other than West Germany, since those channels up to the E69 channel must be able to be received, the constant voltage diode, .mu.PC-574J, having the above described range of the operating voltage cannot be employed. The reason is that in order to be able to receive the channels up to the E69 channel of the UHF band, the margin of .+-.5 MHz is needed in consideration of the above described frequency fluctuation factors. Thus, if an attempt is made to be able to receive the E69 channel with the margin of .+-.5 MHz, the operating voltage of the constant voltage diode 413 must be restricted to approximately 33 V.+-.0.6 V, for example. However, under the existing circumstances, such a constant voltage diode having a narrow width of the operating voltage and various good characteristics such as temperature characteristic has not been put into practical use. Accordingly, under the existing circumstances, this is only the way in which a constant voltage diode such as the above described .mu.PC-574J is selected. It is clear that selection of the operating voltage is not suitable for mass production and is not practical.
On the other hand, the tuning voltage Vt applied to the tuner not only determines the receiving frequency range, but also causes the response and gain of the tuner to change respectively. Tracking adjustment must be made in the tuner so as to obtain a good and stable response and gain over the frequency variation range in each band. Thus, restriction of the tuning voltage Vt to some range, for example, 33 V.+-.0.6 V in making the tracking adjustment remarkably reduced freedom for the tracking adjustment. As a result, even this approach introduces defects that mass production cannot be achieved and freedom for adjustment of the tuner in the course of manufacturing process is restricted. Under the existing circumstances, a tuning voltage generating apparatus has not been obtained which is capable of receiving up to the E69 channel of the UHF band, for example, using a conventional tuning voltage generating circuit of a voltage synthesizer type and in addition, can fully satisfy the standard of FTZ.